In recent years, aqueous polymer compositions have attracted attention as material for converting to a waterborne system from an organic solvent system in the coating field. Waterborne paints produced from aqueous polymer compositions, however, still have not exhibited sufficient physical properties such as water resistance, stain resistance, and hardness in comparison with organic solvent-based paints. In the description here, an aqueous polymer composition indicates the polymer dissolved and/or dispersed in water.
In this field, in order to improve physical properties of a coating film, a functional group is generally introduced in the polymer of aqueous polymer composition to make cross-linkable, so that a coating film composed of a cross-linked product of polymer (hereinafter referred to as cross-linked coating film) can be formed.
As an aqueous polymer composition to form a cross-linked coating film, the demand for cold-curing, a cold-curing, one-pack type which is a mixture of a cross-linking agent and a polymer, capable of forming a cross-linked coating film in association with evaporation of an aqueous medium without heating when applied is highly demanded. In response to the demand, a hydrazone cross-linked waterborne polymer dispersions with use of a dehydration condensation reaction between a carbonyl group and a hydrazide group have attracted attention in recent years.
In the description here, “cold-curing, one-pack type” indicates a paint which forms a coating film at 25° C., having storage stability. In determination of storage stability, for example, when a coating film formed from a paint applied to a substrate after storage at 50° C. for 2 weeks has water resistance, stain resistance, and hardness similar to those of a coating film formed from the paint before the storage, the paint is considered to have storage stability.
For example, a method for providing a waterborne paint having both of the cold-curing ability and the storage stability, excellent in hardness, stain resistance, etc., by adding dicarboxylic acid dihydrazide as a cross-linking agent to a carbonyl group-containing waterborne polymer dispersion, has been proposed (Patent Literature 1 to 6). In this method, however, dicarboxylic acid dihydrazide for use as cross-linking agent is hydrolyzed during storage of a waterborne paint, resulting in lowered cross-linking ability (curing properties). In other words, the ability to form a cross-linked coating film having excellent hardness, stain resistance, and solvent resistance is lowered with time. Furthermore, in the above literature, a compound having low compatibility with a carbonyl group-containing copolymer and high hydrophilic properties such as adipic acid dihydrazide is used as dicarboxylic acid dihydrazide, causing a disadvantage that the produced cross-linked coating film has markedly inferior water resistance.
Since the curing properties of a conventional waterborne paint containing a conventional cross-linking agent and a carbonyl group-containing copolymer deteriorate with time as described above, the composition cannot exhibit adequate cross-linking performance when applied to a substrate surface. Furthermore, use of dicarboxylic acid dihydrazide having low compatibility with a carbonyl group-containing copolymer as cross-linking agent causes a problem that the produced coating film by application has markedly inferior water resistance.
In Patent Literature 7 and 8, at least one selected from the group consisting of a semicarbazide derivative and a terminal-blocked product thereof obtained by a reaction between a polyisocyanate having 3 to 20 isocyanate groups and hydrazine or a derivative thereof, or a mixture of a non-terminal-blocked product and a terminal-blocked product thereof; and a semicarbazide mixture including at least one selected from the group consisting of the semicarbazide derivative and a terminal-blocked product thereof, and at least one selected from the group consisting of a hydrophilic group-containing compound and a terminal-blocked product thereof is proposed as a cross-linking agent. The semicarbazide mixture, however, includes a large amount of compounds with terminal amino groups as by-products, having a disadvantage of yellowing easily caused by a basic substance.
In Patent Literature 9, in addition to the auto-oxidation in a mixed system of an alkyd type emulsion and a styrene-acrylic emulsion containing large amounts of carboxylic acid, a styrene-acrylic emulsion ring-opening epoxy group is supported as a second cross-linking agent for combination use of the cross-linking reaction with dicarboxylic acid dihydrazide. The water resistance of the coating film of acrylic emulsion containing large amounts of carboxylic acid, however, is not satisfactory.
In Patent Literature 10, in order to improve the water resistance of a coating film for exhibiting the stain resistance, use of isophorone disemicarbazide as cross-linking agent in a waterborne polymer dispersion formed of copolymer which contains a carbonyl group having a high Tg (glass transition temperature) is disclosed. The combination of a polymer having a high Tg and isophorone disemicarbazide, however, has a disadvantage of incapability of film formation even with use of a coalescing agent, due to hardening in a film formation process.
Though isophorone disemicarbazide is disclosed in Patent Literature 11, a semicarbazide composition having a large amount of compounds having terminal amino groups is obtained due to use of an excessive amount of hydrazine during reaction, so that use of the semicarbazide composition as cross-linking agent causes a problem that the produced cross-linked coating film easily causes yellowing when immersed in an aqueous alkali solution. Furthermore, the production method described in Patent Literature 11, a step for removing remaining hydrazine is required.